Harmful substances contained in the exhaust gas of an internal combustion engine can be purified or removed by means of a catalyst. It is known that when a large amount of HC flows into this catalyst, HC poisoning will occur. The purification or removal performance of the catalyst is decreased due to this HC poisoning. On the other hand, it is also known that in cases where HC poisoning has occurred, oxygen is supplied to the catalyst thereby to recover the HC poisoning, by making the air fuel ratio of a mixture higher than a stoichiometric air fuel ratio (for example, refer to a first patent literature).
Here, in a lean burn internal combustion engine having a three-way catalyst which is operated at an air fuel ratio higher than the stoichiometric air fuel ratio (hereinafter, also referred to as a lean air fuel ratio), it may be operated at an air fuel ratio lower than the stoichiometric air fuel ratio (hereinafter, also referred to as a rich air fuel ratio). In such an internal combustion engine, HC poisoning may occur in the three-way catalyst during the operation thereof at a rich air fuel ratio. This HC poisoning is recovered when the internal combustion engine is thereafter operated at a lean air fuel ratio. However, the exhaust gas purifying ability of the three-way catalyst is low until the HC poisoning is recovered, so that a part of hydrocarbon (HC) flowing into the three-way catalyst during that period (HC poisoning) may pass through the three-way catalyst without being oxidized therein. Accordingly, in the case where the HC poisoning of the three-way catalyst has occurred, it is desirable to recover the three-way catalyst from the HC poisoning at an early stage.
Note that in the three-way catalyst, on a lower catalyst layer including an oxygen occlusion or storage agent and a precious metal, there may further be disposed an upper catalyst layer including an oxygen storage agent and a precious metal. That is, the catalyst layers are stacked or laminated one over the other. Here, when the internal combustion engine is operated at a lean air fuel ratio in the case where HC poisoning has occurred in the upper catalyst layer and the lower catalyst layer, first of all, the HC poisoning of the upper catalyst layer will mainly be recovered, and thereafter, the HC poisoning of the lower catalyst layer will be recovered. However, it is considered that in the lower catalyst layer, even if the internal combustion engine is operated at a lean air fuel ratio after HC poisoning has occurred, oxygen is first occluded or stored by the oxygen storage agent which has a high reactivity with oxygen, and thereafter, the reaction of HC attached to the precious metal and oxygen occurs. For this reason, in order to recover the HC poisoning of the lower catalyst layer by operating the internal combustion engine at a lean air fuel ratio, it is necessary to wait until oxygen is stored in the oxygen storage agent of the lower catalyst layer, and hence, a certain period of time is required by the time the HC poisoning of the lower catalyst layer is recovered.